The detection of sample fractions is an important part of chemical analyses based on HPLC analysis (HPLC=High Performance Liquid Chromatography) and CE analysis (CE=Capillary Electrophoresis). Two procedures employed to this end are UV-absorption detection and fluorescence detection. It is known to use in the case of UV-absorption detection a light source which emits both UV light and visible light. In the case of fluorescence detection, the sample is irradiated with exciting light, therewith causing the sample molecules to fluoresce in a known manner. It is desirable to be able to perform the detection process in a measuring cell consisting of a narrow tube (capillary), both with regard to measuring absorption and fluorescence. In many cases, it is desired to effect the measuring process on-line, i.e. directly in the capillary tube, which forms a separation column for fractions of the sample medium flowing therethrough.
It is also known to perform UV-absorption detection by transmitting a light beam of appropriate wavelength through the cuvette tube, transversely to its longitudinal axis, and onto a photodetector placed on the opposite side of the tube, where the light intensity is detected. A decrease in light intensity between light source and photodetector will therewith constitute a measurement of the concentration of absorbent substances in the sample medium contained in the cuvette tube.
Different methods of increasing the optical path in absorbency measuring processes are also known to the art; Swedish Patent Specification No. 8100194-3 can be mentioned as an example in this respect. This publication discloses an arrangement in which mutually parallel light beams are emitted from a light source located at one end of a cuvette tube, and led into said tube through the medium of an optical element. Subsequent to its passage through the sample medium in the tube, the light is led out of the tube and light beams, which are parallel with the cuvette tube, are led into a detection device. The arrangement can also be used for fluorescence-detecting purposes.